Papers by Keyword: Creep Strength

Abstract: In recent years, AlCoCrFeNi High-Entropy Alloy (HEA) has attracted attention because it is expected to be a next-generation aerospace engine material because it has specific strength, excellent corrosion and wear resistance. However, the influence of microstructure of the material synthesized via spark plasma sintering (SPS), a powder metallurgy technique, on high temperature mechanical properties, in particular, creep features, has rarely been reported. In the study, to investigate the abovementioned issue, the HEAs using powder mean size of 14.6, 41.9 and 82.4 μm synthesized via SPS at 1273 K and 1373 K were prepared. Creep tests were conducted at 973 K. The obtained results indicated that HEAs SPSed at 1373 K exhibited higher creep strength than those of synthesized at relatively low temperature, because the microstructure of the former is different from those of the latter. In addition, FCC/B2 phase boundary fracture was observed for HEA synthesized at 1373 K. By contrast, powder boundary fracture was observed for the remaining HEAs. Moreover, the Monkman-Grant relation can be employed to predict creep rupture time for all types of HEAs on one master curve.

Abstract: The flexural performance, 24 h creep-24 h recovery of the Populus alba wood fiber reinforced high-density polyethylene composites prepared by forming mat-compression molding method is investigated. Two-parameter power law model, Findley’s power law model and Burgers model are used to describe the 24 h creep curve of wood fiber reinforced polymer composite. Through model testing and parameter testing, and comparing the sum squared error of three models found that four elements Burgers model is successfully simulated with the creep resistance performances of resulted wood fiber reinforced polymer composites, and the two-parameter power law model is the worst. In summary, the four elements Burgers model is suitable for describing the creep performances of wood fiber reinforced polymer composites and predicting the safety and stability during application.

Abstract: Compressive creep behavior of hot-rolled (40%) Mg-Y binary and Mg-Y-Zn ternary dilute solid solution alloys are investigated in this study. Creep strength is substantially improved by the addition of zinc. Activation Energy for creep in Mg-Y and Mg-Y-Zn alloys are around 200 kJ/mol at the temperature range from 480 to 570 K. These values are higher than the activation energy for self-diffusion coefficient in magnesium (135 kJ/mol). Many stacking faults, which are planar type defects are observed on the basal planes of the magnesium matrix in Mg-Y-Zn ternary alloys. TEM observation has been revealed that the non-basal a-dislocation slip is significantly activated by these alloys. The rate controlling mechanism of Mg-Y and Mg-Y-Zn dilute alloys are considered to the cross-slip or prismatic-slip controlled dislocation creep with high activation energy for creep, more than 1.5 times higher than the activation energy for creep controlled dislocation climb.

Abstract: The estimation of the lifetime for equipments and installations working at high temperatures represent an actual problem. In the last years, there have been proposed a lot of methods in order to evaluate the lifetime for the equipments working under creep conditions. In the frameworks of the paper, some results regarding the behavior of pipes belonging to a methane gas cracking reactor are presented. Pipes worked on about 160.000 hours under a pressure of 14 at and a temperature of 800°C. The behavior of the pipes under above mentioned pressure has been calculated and plotted. Creep tests were performed at 650 and 800°C, and on these bases was evaluated the creep strength of the material. With Larson-Miller method the results were prolonged for spans shorter than 10.000 hours. The creep strength variation curves, drawn for 1000 and 100000 hours can be used for predictions about the lifetime at different.

Abstract: We examine the mechanical and physical properties of Sanicro 25 steel that are relevant to the performance requirements of supercritical boilers (e.g., resistance to high-temperature corrosion). Sanicro 25 is mainly used in the construction of power plant components. Materials research has demonstrated the stability of properties at elevated temperatures (700 to 750°C) during long-term use. We conducted tests of corrosion resistance for Sanicro 25 steel, and confirmed its resistance to oxidation in steam and exhaust gas heat .

Abstract: Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M₂₃C₆ precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental overshoot in temperature during PWHT. The short excursion to high temperature beyond Ac₃ would have resulted in the formation of deleterious phases, for example, soft α-ferrite which has poor creep strength and hard martensite which has a low toughness. In this study, the degraded specimens with soft α ferrite as a result of cooling transformation from 900°C are proven to have a limited creep rupture life where the creep rupture strength dropped remarkably after 1000 hours. As the peak temperature increased to 950°C and 1000°C, hard and brittle martensite was formed on cooling. The creep specimens were found to exhibit better creep strength; most probably the creep behavior was improved by the tempering effect at 600°C during creep tests. Nevertheless, despite the tempering which might have improved the toughness slightly, the high temperature creep rupture stress still had dropped approximately 40%, as compared to the virgin alloys in the range of rupture time from 1,000 hours to 10,000 hours.

Abstract: Ferritic heat resistant steels are strongly desired to expand their maximum use temperature up to 650°C for application to the next highest temperature components of highly efficient, low emission ultra-supercritical (USC) power plant with maximum steam temperature of 700 °C. This minimizes the requirement of expensive nickel base superalloys. Critical issues for the development of ferritic steels for 650 °C USC boilers are the improvement of oxidation resistance as well as long-term creep rupture strength, including welded joints. The optimized combination of boron addition and dispersion of nanometer-sized vanadium nitrides significantly improves long-term creep strength of 9% Cr steel and also exhibits no degradation in welded joints at 650 °C. The protective Cr₂O₃-rich scale forms on the surface of 9% Cr steel by pre-oxidation treatment in Ar gas, which significantly improves the oxidation resistance in steam at 650°C.

Abstract: The microstructures of thixomolded^® (TM) Mg-Al-Ca alloys consist of α-Mg and eutectic compounds along grain boundaries. Misch metal (Mm) addition to TM Mg-Al-Ca alloys makes precipitates within α-Mg matrix and their number density and size depend on heat-treatment conditions. The small addition of Mm can keep the network-like grain boundary covering and the improvement of microstructure stability during creep. On the other hand, excessive Mm addition causes the deterioration of creep strength. The grain boundary coverage decreases with increasing Mm content due to the formation of coarse spherical Al-Mm based intermetallic compounds. Creep strength is significantly affected by both of the grain boundary coverage and the morphology of eutectic compounds along grain boundaries.

Abstract: The microstructure and creep strength of 403Nb martensitic heat-resistant steel after tempering at 650°C, 680°C and 730°C were analyzed by SEM and TEM. Microstructural observation shows that 403Nb steel tempered at 650°C provides a finer lath structure and a smaller size of precipitates than that tempered at 680°C or 730°C. The dislocation density is also higher in 403Nb steel tempered at 650°C. The creep rupture strength of 403Nb steel tempered at 650°C is the highest among those tempered. The main creep strengthening mechanisms in 403Nb steel usually include precipitates hardening, dislocation hardening and sub-grain boundary hardening.

Abstract: Refractory materials, in particular tungsten base materials are considered as primary candidates for structural high heat load applications in future nuclear fusion power plants. Promising helium-cooled divertor design outlines make use of their high heat conductivity and strength. The upper operating temperature limit is mainly defined by the onset of recrystallization but also by loss of creep strength. The lower operating temperature range is restricted by the use of steel parts for the in- and outlets as well as for the back-bone. Therefore, the most critical issue of tungsten materials in connection with structural divertor applications is the ductile-to-brittle transition. Another problem consists in the fact that especially refractory alloys show a strong correlation between microstructure and their manufacturing history. Since physical and mechanical properties are influenced by the underlying microstructure, refractory alloys can behave quite different, even if their chemical composition is the same. Therefore, creep and thermal conductivity have been investigated using typical commercial tungsten materials. Moreover, the fracture behavior of different tungsten based semi-finished products was characterized by standard Charpy tests which have been performed up to 1100 °C in vacuum. Due to their fabrication history (powder mixing, pressing, sintering, rolling, forging, or swaging) these materials have specific microstructures which lead different fracture modes. The influence of the microstructure characteristics like grain size, anisotropy, texture, or chemical composition has been studied.